If you had a custom tune for the 383 it may be worth checking the cranking, after start, and warm up power enrichment. I had a 24x conversion done and had a professional tuner do the base and dyno tune based on a 2002 silverado with an LM7. After done the truck ran great warm but had an issue that for the first 90 seconds or so after a dead cold start (as in 8 hours off minimum, any residual warmth in the motor and it wouldn't happen) it would start and idle fine but off ide throttle would result in a very lean condition causing a near stall for about 3 seconds and then surging to power. I bought HP Tuners primarily to have a base tune should the ECM ever fail but also to play with trying to fix the cold warm up condition myself (the tuner said he couldn't replicate it). What I found was that the OL after start and transient warm up enrichment tables were not updated so still had stock LM7 tables which, looking at the numbers, heats up quite a bit faster than an L31 given that it uses a little less enrichment for a significantly shorter duration. Copied in the L31 stock tables along with the warm-up throttle set-points and the cold performance surge went away. If you haven't had a tune it would be worth getting one as I'm certain a 383 won't run the same as an L31. Maybe check the ECT during start-up/warm-up with a scanner as well just to ensure it's giving good signal to the ECM.